Bt toxins are

Options

A. intracellular lipids
B. intracellular crystalline proteins
C. extracellular crystalline proteins
D. intracellular polysaccharides

Answer

**Step-by-Step Solution:**

1. **Identify the Source of Bt Toxins**: Bt toxins are produced by the bacterium *Bacillus thuringiensis*.

2. **Nature of Bt Toxins**: These toxins are proteins, specifically designed to target certain insects.

3. **Function of Bt Toxins**: The primary function of Bt toxins is to kill specific insect pests, including those from the orders Lepidoptera, Diptera, and others.

4. **Form of Bt Toxins**: The *Bacillus thuringiensis* bacterium produces these toxins in the form of intracellular crystalline proteins.

5. **Activation of Bt Toxins**: The Bt toxins are initially inactive and become activated when they are ingested by the target insect. The alkaline pH of the insect's gut solubilizes the crystalline form, converting it into an active toxin.

6. **Mechanism of Action**: Once activated, the toxin binds to the midgut epithelial cells of the insect, creating pores that lead to cell swelling, lysis, and ultimately, the death of the insect.

7. **Conclusion**: Based on the information, the correct answer to the question "Bt toxins are" is **option 2: intracellular crystalline protein**.

---